1. Field of the Invention
The present invention relates to an area flow meter for measuring a flow rate of a fluid passing through a flow passage pipe on the basis of a position of a float movably mounted inside the flow passage pipe.
2. Description of the Prior Art
A so-called area flow meter is widely used, in which a movable element or float is larger in specific gravity than a fluid being measured. The float is disposed inside a vertical flow passage pipe of the flow meter so as to be movable up and down therein. As the float moves upward, the flow passage pipe has its effective cross-sectional area gradually increased. A flow rate of a fluid passing through the flow passage pipe upward is determined on the basis of the position of the float. Some of the area flow meters have their flow passage pipes constructed of opaque members. In this opaque type of area flow meter, some detection means which is disposed outside the flow passage pipe so as to detect the position or height of the float outside the pipe is required. It is necessary for such detection means to convert a value of the position or height of the float into an electrical signal.
There are various types of the detection means for converting the value of the height of the float into the electrical signal. As shown in FIG. 8 (Prior Art), the applicant's company has developed one of these conventional detection means for determining the height of the float, in which one: a magnet M is fixedly mounted in a float F; and, a pair of magnetic sensors such as Hall devices A-1, A-2 are disposed outside the flow passage pipe so that the height of the float is determined on the basis of output signals issued from the magnetic sensors such as the Hall devices.
In the conventional detection means, the height of the float is determined on the basis of the angle of inclination of the magnetic-force line issued from the magnet mounted in the float. The conventional detection means requires no movable mechanism with the exception of the float, and, therefore is excellent in reliability and compact in construction. However, the conventional detection means is suffering from the following problems:
(1) Since the magnet is largely spaced apart from the magnetic sensors, the magnetic force of the magnet damps or weakens in the vicinities of the magnetic sensors. Consequently, in order to ensure high-accuracy measurements of the position of the float, it is necessary for the conventional detection means to have its magnetic sensors improved in sensitivity or to have its magnet improved in strength of magnetic force; PA0 (2) In the area flow meter, there is a close relationship between dimensions/weight of the float and the measuring range of the flow meter, which limits the magnet mounted in the float in size, and, therefore in strength of magnetic force; PA0 (3) Of the conventional magnetic sensors, ones excellent in sensitivity and stable in output are very expensive and not available for reasonable prices, which makes it difficult to use such expensive sensors in the conventional area flow meter; and PA0 (4) Some fluid to be measured by the area flow meter is high in temperature. When such high-temperature fluid is measured in flow rate, it is necessary to have the flow passage pipe or the float largely spaced apart from the magnetic sensors so as to permit the magnetic sensors to be disposed in places having moderate temperatures which ensure proper operations of the magnetic sensors. However, this arrangement of the magnetic sensors is not realistic due to poor sensitivities of the magnetic sensors. Further, there are various types of magnetic sensors which permit their electrical variables representing physical quantities to vary in response to the magnetic flux density of the magnet. Of these types of magnetic sensors, a typical one is a so-called Hall device.
As shown in FIG. 9 (Prior Art), the Hall device is provided with four terminals, of which a pair of ones are electric-power supply terminals. When the Hall device is energized with an electric power P supplied through the electric-power supply terminals, the other pair of the terminals of the Hall device produce there between an output voltage E which corresponds to a magnetic flux density B of the magnet, which magnetic flux density B is measured in a magnetism-responsive surface of the Hall device. Consequently, it is possible to determine the magnetic flux density B of the magnet on the basis of the output voltage E of the Hall device, the magnetic flux density B appearing in the magnetism-responsive surface of the Hall device.
In the conventional detection means shown in FIG. 8 described above, the pair of the Hall devices A-1, A-2 have their magnetism-responsive surfaces crossed at right angles with each other so that the position or height of the magnet in the float is determined on the basis of output voltages issued from the Hall devices A-1, A-2. This conventional detection means is excellent in principle. However, the detection means requires the Hall devices to be excellent in output sensitivities corresponding to the magnetic flux density in order to realize high-accuracy measurements of the fluid in flow rate. As a means for improving the Hall device in output sensitivity, there is a known amplifier means for electrically boosting the output voltage of the Hall device. However, such amplifier means also boosts any other noises such as electrical noises of the Hall device itself, external electrical noises, and other electrical physical quantities forming obstacles to the measurements of the fluid in flow rate, the obstacles including the zero drift of the Hall device in the measurements. Consequently, the amplifier means is not adequate to the needs.
Further, there are various methods for catching the magnetic flux of the magnet through magnetic-permeability elements excellent in permeability. However, any of these methods is not good in application thereof.
The reason why an effective application of any one of the above methods is not realized in the prior art is that: the magnetic-permeability element excellent in permeability tends to be magnetically saturated, which prevents the magnetic flux proportionate to the magnetic flux density near the element from concentrating in the element.